Remote plasma enhanced CVD method and apparatus for growing an epitaxial semiconductor layer

ABSTRACT

A remote plasma enhanced CVD apparatus and method for growing semiconductor layers on a substrate, wherein a intermediate feed gas, which does not itself contain constituent elements to be deposited, is first activated in an activation region to produce plural reactive species of the feed gas. These reactive species are then spatially filtered to remove selected of the reactive species, leaving only other, typically metastable, species which are then mixed with a carrier gas including constituent elements to be deposited on the substrate. During this mixing, the selected spatially filtered reactive species of the feed gas chemically interacts, i.e., partially dissociates and activates, in the gas phase, the carrier gas, with the process variables being selected so that there is no back-diffusion of gases or reactive species into the feed gas activation region. The dissociated and activated carrier gas along with the surviving reactive species of the feed gas then flows to the substrate. At the substrate, the surviving reactive species of the feed gas further dissociate the carrier gas and order the activated carrier gas species on the substrate whereby the desired epitaxial semiconductor layer is grown on the substrate.

This invention was made with U.S. Government support under contactN00014/84/C/0659 awarded by the U.S. Department of the Navy. The U.S.Government has certain rights in this invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.07/375,949 filed Aug. 10, 1989, now U.S. Pat. No. 5,018,479, which is adivisional application of U.S. application Ser. No. 07/100,477 filedSep. 24, 1987, which issued as U.S. Pat. No. 4,870,030.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a remote plasma enhanced chemical vapordeposition (RPECVD) apparatus and method for growing an epitaxialsemiconductor layer.

2. Discussion of Background

Plasma enhanced processes have figured prominently in research effortsto lower process temperatures. In conventional plasma enhanced chemicalvapor deposition (PECVD), the parent gas molecules are dissociated intoprecursor atoms and radicals which can deposit on substrates at lowertemperatures than in thermal chemical vapor deposition. The depositionoccurs at lower temperatures than purely pyrolytic processes because theplasma supplied energy to break chemical bonds in the parent moleculesthat would only be broken by thermal decomposition if the plasma werenot present. Parent molecule dissociation is accomplished in the plasmathrough various processes involving collisions with electrons, ions,photons, and excited neutral species. Unfortunately, the precursorspecies are also subject to the same active environment whichdissociated the parent molecules. This can lead to further dissociationor reaction of gas phase species to form more complicated radicalsbefore the radicals can condense on the substrate. In a low pressure,low power silane (SiH₄) immersion plasma, Matsuda et al. Thin SolidFilms 92,171 (1982), have shown using mass spectroscopy that there are ahost of gas phase species. These species include H, H₂, Si, SiH, SiH₂,SiH₃, SiH₄, Si₂, Si₂ H, Si₂ H₂, Si₂ H₃, Si₂ H₄, and Si₂ H₅. The mostdominant line in the mass spectroscopy is the SiH₂ line, even though itis ony 12% taller than the SiH₃ line and 125% taller than the Si₂ H₅line. There is a wide spectrum of precursor species incident on thegrowing film. A further complication is that in conventional PECVD thesubstrate is immersed in the plasma region. This results in a large fluxof charged species incident on the substrate during film deposition. Theincident energies of these ions may be as high as 160 eV in someimmersion systems (See Chapman, Glow Discharge Processes, John Willey &Sons, N.Y. 1980, Chap. 4). This can lead to ion implantation, energeticneutral embedment, sputtering, and associated damage. This residualdamage must be annealed out during growth if high quality epitaxiallayers are to be produced. Thus, this damage imposes a minimum growthtemperature, based on annealing conditions below which high qualitymaterial cannot be obtained. Thus, there are two major problemsassociated with conventional PECVD: adequate control over incident gasphase species, and ion damage as a result of the substrate beingimmersed in the plasma region.

RPECVD deposition of silicon nitride Si₃ N₄ and silicon SiO₂ for gateinsulators in (In, Ga) As FET devices has recently been disclosed byRichard et al. J. Vac. Sci. Technol. A3(3), May/June 1985 (pages867-872). According to this reference, to deposit SiO₂, for example, onereactant, O₂, is excited in the plasma tube remote from thesemiconductor substrate. The other reactant, SiH₄, enters the reactorseparately, near the substrate and is not excited to a plasma state. Animportant point is that one of the reactants, O₂, bearing one of thecomponent atoms of the SiO₂, is introduced through the plasma tube. Theprocess is thought to follow the following reaction model. Monosilane(SiH₄) molecules interact with the metastable oxygen O*_(x) (³ P_(j))flux resulting from the remote plasma. The lifetime of the metastableoxygen is quite long, allowing pathlengths of 1-2 meters in the RPECVDreactor using the SiO₂ deposition parameters. (In contrast, thepathlength of a typical metastable excited noble gas specie, e.g. He*,used in the RPECVD epitaxial growth of semiconductor layers, accordingto the present invention, is 5-30 cm.) This interaction leads todisiloxane, (SiH₃)₂ O, formation in the gas phase. On the heatedsubstrate, disiloxane is further oxidized by excess metastable oxygen,O*. This oxidation removes H from the silyl groups, SiH₃.Dehydrogenation is accompanied by oxygen bridging of silicon atomsoriginally bound in adjacent disiloxane molecules on the heated surface.An excess of the plasma excited species is used to drive thedehydrogenation of the silyl groups to completion, minimizing Si--Hbonding. Silicon-poor films do not form; thus the process is stable. Forthis case, CVD can be thought of as a polymerization of disiloxanebrought about by oxidation of the SiH bonds of the silyl groups.

Important features of the SiO₂ process described by the above-notedRichard et al article are:

1) In the SiO₂ process, O is activated by the plasma in the plasmageneration region and becomes incorporated in the deposited layers.

2) The interaction between the reactive species existing the plasmageneration region and the injected reactant results in the formation ofthe chemical groups.

3) The lifetimes and therefore the pathlengths of the reactive speciesexiting the plasma formation is quite long: for metastable oxygen thepathlength is 1-2 meters.

4) The dielectric material formed, SiO₂, is an amorphous material andtherefore has no long-range or crystalline order. For SiO₂ deposition,metastable O* promotes the further oxidation of disiloxane adsorbed onthe substrate surface, which reduces the surface of adatoms and enhancesthe formation of amorphous material.

Another prior art reference of interest is an article by Toyoshima etal, Appl. Phys. Lett. 46(6), 15 March 1985, pp 584-586, which describesa PECVD process to deposit hydrogenated amorphous silicon. However, thedeposited a-Si:H films retain from 5-30 atomic percent hydrogen in thedeposited layers, which is critical to the performance of a-Si:H, butdisastrous if one is trying to grow epitaxial Si layers. No process usedto deposit high quality a-Si:H films has proven successful in depositingepitaxial Si layers.

SUMMARY OF THE INVENTION

Accordingly, one object of this invention is to provide a new andimproved apparatus and method for growing epitaxial semiconductor layerson a substrate, which overcomes the problems in the prior art PECVDtechniques above-noted, including inadequacy of the control overincident gas phase species, ion damage to the substrate, and the lack ofexcited metastable gas species at the substrate to enhance surfacemobility of the adatoms and formation of the epitaxial layer.

Another object of this invention is to provide a novel apparatus andmethod employing an improved RPECVD approach, by which epitaxialsemiconductor layers can be deposited on a substrate maintained at arelatively low temperature.

Still a further object of this invention is to provide a novel RPECVDapparatus and method for growing epitaxial diamond layers on asubstrate.

These and other objects are achieved according to the invention byproviding a new and improved RPECVD apparatus and method for growingsemiconductor layers on a substrate wherein an intermediate feed gas,which does not itself contain constituent elements to be deposited, isfirst activated in an activation region to produce plural reactivespecies of the feed gas. These reactive species are then spatiallyfiltered to remove selected of the reactive species, leaving only other,typically metastable, reactive species which are then mixed with acarrier gas including constituent elements to be deposited on thesubstrate. During this mixing, the selected spatially filtered reactivespecies of the feed gas chemically interacts, i.e. partially dissociatesand activates, in the gas phase, the carrier gas, with the processvariables being selected so that there is no back-diffusion of gases orreactive species into the feed gas activation region. The dissociatedand activated carrier gas along with the surviving species of the feedgas then flows to the substrate. The surviving reactive species of thecarrier gas completely react and the surviving metastable specie of thefeed gas completely order the activated carrier species on the substratewhereby the desired epitaxial semiconductor layer is grown on thesubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic cross-sectional side view of a flow tubeschematically illustrating key features of the present invention; and

FIG. 2 is a schematic side view illustrating in more detail a RPECVDreaction chamber used according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, it isfirst noted that the reactor design and operating criteria discussedhereinafter are based on the principle of a remote region of activationof a gas or mixture of gases. The activated gas (gases) then playsseveral roles leading to the deposition of the semiconducting film.Because of the central importance of "remote region of activation" tothe present invention, this terminology is first defined referring toFIG. 1.

FIG. 1 shows schematically an intermediate section of a flow tube 12. Afeed gas stream (single gas, vapor, or mixture) enters at input inlet10. In the region of activation 14, the feed gas has its chemicalreactivity increased. Chemical reactivity of the feed gas can beincreased in many ways. For example, one or more components of the feedgas may be ionized; one or more components of the feed gas may bedissociated into more reactive species, such as converting water vaporinto hydrogen and oxygen; or the internal energy of the feed gas may beincreased without ionization. This can be accomplished by many methods.Some of these methods can be internal to the flow tube. A sample ofthese internal methods might include heaters, or catalytic surfaces, andelectron or ion bombardment sources. Some methods could be external tothe flow tube. A sample of these external methods might include a broadrange optical sources (with an appropriately transparent tube),microwave or radio frequency power sources, or simple heaters. Whateverthe feed gas(es), the combined means for activation, or the reactivespecies formed, in the activation region 14, energy is coupled into oneor more gases, and that energy can contribute to subsequent chemicalreactions.

In experimental studies performed to date, an external radio frequencycoil 14₁, shown in FIG. 2, concentric with the flow tube was used toactivate the gas stream.

Referring to FIG. 1, the concept of a "remote" region of activation inthe present RPECVD technique will be described. By remote region ofactivation is meant two things: (1) the substrate is not located in aremote region of activation; (2) in any remote region of activation,only gas(es) from the inlet of that region of activation is (are)present, other gas(es) that may be present in other regions of theapparatus can not reach a remote region of activation by diffusion orother processes that would allow such gas(es) to enter through the exitof a region of activation. To ensure this requires both a suitablereactor design and a proper selection of operating parameters. In theflow system of the present invention, shown in FIGS. 1 and 2, the designof the physical separation of the various regions of the reactor,coupled with the flow velocity of the gas stream (which of coursedepends on the selection of process parameters) in those regions,determines whether back-diffusion of gases into a region of activationcan occur.

As shown in FIG. 1, the present invention includes a feed gas inlet 10through which a feed gas is entered into a plasma tube 12. In anactivation region 14, the chemical reactivity of the feed gas isincreased to produce reactive species of the feed gas which passdownstream of the exit plane 14₂ in the downward direction shown inFIG. 1. Between the exit plane 14₂ and the carrier gas inlet 18, thefeed gas reactive species are filtered such that only the desired speciereaches the gas including a constituent element to be deposited inlet 18where it mixes and interacts with a carrier gas introduced via the inlet18 in a mixing and interaction region 20.

In a working embodiment of the invention used to date, a radiofrequencycoil 14₁ concentric with the flow tube 12 has been used to create a"plasma" (glow discharge) of the feed gas in the activation region 14.Working examples have used either a pure noble gas plasma feed, asdiscussed hereinafter, or noble gas mixtures with hydrogen. The plasmaenvironment in the activation region 14 contains many species, even witha simple feed gas like helium. In fact, the feed gas reactive speciesproduced in the activation region 14 include ions, electrons, and a hostof excited species all with different composite lifetimes which areinfluenced by various factors. The flow through the activation region 14carries the species downstream towards the carrier gas inlet 18 and asubstrate 22 mounted in a deposition region 24 downstream of the inlet18. The distance that the various species can travel before they areannihilated will depend on their composite lifetimes and the flowvelocity. According to the invention, the flow velocity of the feed andcarrier gases are controlled so as to control the relative abundance ofselected of the reacted species at a given distance downstream of theregion of activation, such as at the mixing and interaction region 20 atthe carrier gas inlet 18. Thus, by controlling the gas flow rates, andby requiring the reactive species of the feed gas to pass from the exitplane to the mixing and interaction region 20, a spatial filteringregion 26 is provided downstream of the exit plane 16, in whichundesired reactor species are annihilated and only selected of thereactive species are passed downstream towards the mixing andinteraction region 20.

Spatial filtering as above described involves two aspects. First, somephysical distance between the activation region which excites the feedgas and the region where the carrier gas is introduced must exist. Andsecond, the lifetimes of the desired reactive species must besubstantially longer than the lifetimes of those species not desired.Once these criteria are established, spatial filtering occurs becausethe pumping velocity of the reactive species determines how fardownstream from the activation region various species will travel beforethey decay or be annihilated. For example, in a He discharge, electronimpact excites He atoms into a host of excited electronic states. Thesestates include 2³ P, 2¹ P, 3³ S, 3¹ S, 3¹ P, 3³ P, 3¹ D, and 3³ D. Allthese states all have energies greater than the metastable 2³ S state.However, these states quickly decay to the ground state or one of thelower metastable states, 2¹ S or 2³ S exponentially with acharacteristic decay time. This decay time is less than 10⁻¹ s. As thespecies are pumped from the discharge regions, the metastables andground state He atoms are dominant. Of the two metastable states, the 2¹S state has the shorter decay time or effective lifetime 2×10⁻⁸ sec vs6×10⁻³ sec for the 2³ S state. Thus, the host of highly excited Hestates in the plasma region have been spatially filtered to produce adesired flux of metastable 2³ S He atoms at the entrance to the gasmixing region. The unwanted excited species are completely attenuatedexponentially along the length of the spatial filter compared to afactor of 3-150 attenuation (for plug velocities of 10-50 m/sec andspatial filter length of 0.3 m) for the desirable metastable specie. Forthis particular spatial filter design and system operating parameters,all activated gas feed species having effective lifetimes less than4×10⁻³ sec will be completely annihilated in the spatial filter.

The spatial filtering region 16 also acts as a backstreaming isolationregion which in conjunction with the selected gas flow rates preventsinjected carrier gas from the inlet 18 from back diffusing to the exitplane of 14₂ of the activation region 14.

The flux of activated noble gas species (and by activated it isspecifically meant in the sense of internal energy and not kineticenergy) partially dissociates and activates (in the gas phase) thecarrier gas. The flux of the activated noble gas species completelyreacts and orders the activated carrier species onto the substrate 22and results in the growth of an epitaxial semiconductor on thesubstrate. The flux of activated spatially filtered noble gas speciesenhances surface reactivity and reactant surface mobility in the growthof a single crystal epitaxial layer. The technique of the invention asapplied to surface effects can be used in a low pressure process wherethe mean free path between the exit plane 14₂ of the activation region14 and the substrate 22 is such that no gas phase collisions occur.

Three examples of specific semiconductive materials grown using theRPECVD technique according to the invention are next discussed. In theseexamples, there is no attempt to limit the invention to these specificfeatures of remote region excitation technique, remote region feed gas,reactant feed gas, or specific reactor system design.

In all three examples, reference is made to a schematic of a remoteplasma enhanced chemical vapor deposition reactor, shown in FIG. 2. Thisrepresentation of a RPECVD reactor primarily consists of a plasma tube12 in which is located the region of activation 14, and an activationsource such as an rf coil 14₁. The plasma tube 12 feeds into adeposition chamber 20₁ in which is located a gas dispersal ring 18, andthe substrate susceptor 28. Additional components include an electrongun 30, phosphorous screen 32, and a manipulator arm 34 used to performReflection High Energy Electron Diffraction (RHEED) characterizations ofthe substrate 22 and the epitaxial semiconductor film deposited thereon.The plasma tube 12 used consists of a 7.6 cm inside diameter pyrex tube.The plasma is driven by a 13.56-MHz rf generator with matching network.The substrates 22 are clamped to a graphite susceptor 28 heatedinternally by a tungsten halogen lamp (not shown). Substratetemperatures are calibrated using thermocouples (not shown) attached tothe surface of a silicon substrate. Gasses are introduced through twoseparate gas feeds, the plasma feed gas inlet 12 and the carrier gasfeed 18₁ to the gas dispersal ring 18, which serves as the carrier gasfeed inlet. The plug velocity of He or other noble gas through the 7.6cm plasma tube 12 is high enough to prevent back-diffusion of GeH₄,SiH₄, or CH₄. The plug velocities used are 3, 5, and 100 m/s forgermanium, silicon, and diamond depositions, respectively. Also shown isan outlet 36 for high vacuum pumping via a turbomolecular pump (notshown), an outlet 38 for pumping the process gasses using a roots blower(not shownclaim) together with a direct drive mechanical pump (notshown). Typical pressures are less than 5×10⁻¹⁰ Torr minimum basepressure when the process gasses are not flowing and 1-300 mTorr duringepitaxial growth of a semiconductor layer. The vacuum intake to theroots blower is ballasted with a constant gas load to preventantibackstreaming of oil vapors. Examples illustrating use of thisprocess to epitaxially grow silicon, germanium, and diamondsemiconductor layers are described below.

Epitaxclaimial growth of germanium is accomplished by flowing 200 sccmof He through the plasma tube and 20 sccm of 0.1% GeH₄ in He through thegas dispersal ring 18. The pressure is controlled at 200 mTorr. Toinitiate deposition, 100 W is applied to the rf coil creating a Hedischarge plasma in the activation region 14. The substrate temperatureis typically maintained between 225°-450° C., preferably at 300° C.,during growth.

Epitaxial growth is thought to occur through the following processes.The rf energy coupled to the plasma tube establishes a He plasma in theactivation region 14. Through a variety of reactions many differentspecies of excited He atoms and ions are created in the plasma, eachhaving its own lifetime. These various species are caused to flow downfrom the plasma tube toward the gas dispersal ring 18 and the substrate.Each specie can be annihilated through a variety of mechanisms, andtherefore, each specie has an average time that it can survive, oreffective lifetime, until it is annihilated. This lifetime can betranslated into an average distance it will travel below the plasma tubeexit plane 14₂ before it is destroyed. This distance is called thepathlength. The pathlength of a specie is determined by the effectivelifetime of the specie and the plug velocity of the He gas flow.Consequently, the system and the growth parameters can be designed andchosen to cause undesired species to be spatially filtered in thespatial filtering region 16 and the desired specie to interact with thereactant molecules and to arrive at the substrate surface. In thepresent example the specie desired to interact with the reactant GeH₄ isthe metastable He(2 ³ S). These metastables play three important rolesin the overall growth process:

1. They dissociate the germane molecules through inelastic gas phasecollisions;

2. They have inelastic collisions on the growth surface of the filmwhich enhances the surface mobility of the impinging species leading toepitaxy at low surface temperatures; and

3. They can also play a role in dehydrogenation of surface reactants.

These three functions for the metastables will be discussed in furtherdetail below.

The metastable He(2 ³ S) interacts with the GeH4 through inelastic gasphase collisions, and creates several reaction products. These productsmay include ionized and neutral GeH_(x) species, where 0<x≦4. The mostprobable products are GeH₄ +, GeH₃ + and GeH₃ ; and the desirableproduct is GeH₃. As the radicals condense on the substrate they mustcross-link to form a germanium network. If this process is to formepitaxial layers of germanium, excess hydrogen carried by the freeradicals must be liberated and the reactant species must have sufficientsurface mobility to form an ordered solid. In the RPECVD processhydrogen removal occurs when He metastables collide with the growthsurface.

Epitaxial growth of silicon proceeds much in the same manner as growthof germanium. Again, growth is accomplished by flowing 200 sccm of Hethrough the plasma tube 12 and by flowing 100 sccm He and 1 sccm SiH₄through the gas dispersal ring 18. A rf discharge plasma of 30 W issustained during deposition. The deposition process occurs at a totalpressure in a range of 50-300 mTorr, with 200 mTorr being preferred. Thedeposition rate is approximately 0.01 nm/s on a Si(100) 1×1 surface at520° C. Epitaxial growth has been achieved at temperatures as low as200° C. with best results occurring at about 400° C. The role of themetastable He in the epitaxial growth of silicon is thought to be muchthe same as described above for the epitaxial growth of germanium.

Epitaxial growth of diamond may be accomplished by flowing a Noble gas(He, Ar, or Xe) through the plasma tube and methane, CH₄, through thegas dispersal ring. One important factor that distinguishes growth ofdiamond from growth of either germanium or silicon is the poly-phasicnature of the deposited material. Depending upon the growth conditions,the deposited layers may be diamond, graphite, amorphous or glassycarbon, or mixtures of these materials. When a hydrocarbon such asmethane is excited in a plasma, radicals of the form CH_(x) are formed.As in the silane example, these radicals interact in the gas phase toform carbon-carbon bonds. The added complication in the carbon caseresults from the ability of carbon to form not one, but threehybridizations. Thus we get carbon-carbon bonding of the ethane form(sp³ hybridization), of the ethylene form (sp² hybridization), and ofthe acetylene form (sp hybridization). The parallel between these gasphase precursors and their solid phase analogues is striking. Diamond(sp³ hybridization) has ethane type bonding, graphite (sp²hybridization) has ethylene type bonding, and carbynes (sphybridization) are chainlike compounds with acetylene type bonding. Togrow semiconducting diamond it is necessary to preclude theincorporation of wrong bonds of graphite-like or carbyne-likehybridization. The flux of gaseous precursors with incorrecthybridization onto the film surface is inevitable if the undesirablemethane radicals (i.e., the ethylene and carbyne) are formed.Consequently, the design of the growth reactor and the choice of thegrowth parameters must be properly chosen to form precursors which uponcondensation on the substrate promote sp³ hybridization and diamondgrowth.

The growth of diamond proceeds technically in a similar way as doessilicon and germanium. Typically, 500 sccm of He flows through theplasma tube 12 with a 30 sccm dilute mixture of He, H₂, and CH₄ (4:10:1by volume) flowing from the gas dispersal ring 18. A rf discharge of 80W is sustained in the activation region 14 during deposition at a totalpressure range of 10-1000 mTorr, typically less than 100 mTorr. Thesubstrate temperatures is varied from 650°-850° C. The quartz plasmatube size is 1.5 in. o.d. insuring a high plug velocity necessary fortransporting metastables and radicals to the substrate. Using thesegrowth parameters, diamond films have been grown at the rate ofapproximately 2000 Å/hr.

While the process technically is very similar to the silicon andgermanium growth, the proper choice of noble gas and methane diluent iscritical for promoting diamond growth. Because the energy of the Hemetastable is so high (˜20 eV), the cross-section for collisionaldissociation of the CH₄ molecule is low. Thus, the depositionalprecursor species created by the He are CH₄ +, CH₃ +, or CH₃, all ofwhich are highly saturated CH_(x) radicals. Also the choice of methanediluent becomes more pertinent. Unlike the growths of silicon andgermanium where the silane and germane were diluted in He, diamondgrowth is more facilitated with hydrogen dilution. The hydrogen servestwo roles. First as a source of atomic hydrogen to the nucleating film,it more preferentially etches the graphitic bonds than the diamondbonds. Second, it moderates the gas phase chemistry promoting highersaturation of the CH_(x) radicals.

In the growth of diamond, H₂ is used in surface reactions such as theetching of graphitic bonding units and in gas phase reactions to convertsp and sp₂ bonded hydrocarbon radials to sp₃ bonding forms. For thispurpose the H₂ source gs is activated by inversion to atomic hydrogen,H(H₂ +energy→2H). In the basic reactor design shown in FIG. 2 thisactivation is carried out in one of two ways. By one method, Case 1, theH₂ gas enters with the carrier gas stream through the gas disposal ring18. The H₂ is activated through interaction with energetic species ofthe feed gas that have passed through the spatial filter 14. Becausethis technique relies on energetic species of the feed gas (ex.metastable He (2³ S)), to activate both the hydrogen and the methane,dramatic reduction in deposition rate is observed as H₂ :CH₄ flow ratiosare increased beyond 20:1. This limitation places a severe restrictionon the range of gas mixtures for which reasonably efficient diamonddeposition rates can be achieved (rates ≧1 Å/sec).

The second method (Case 2) of operation seeks to overcome thislimitation. The H₂ is introduced with the noble gas feed through theplasma region. This allows direct activation of the H₂ (H₂ +energy→2H)by the plasma. However if this scheme is used at powers typical for purenoble gas plasmas (80 W), dramatic reduction in the production of noblegas metastables by the plasma is observed. This is due to the fact thatH₂ dissociation occurs at lower energies than He metastable production.Thus the presence of H₂ shifts the electron energy distribution of theplasma region to lower energies where metastable noble gas production isinefficient. If it is attempted to overcome this limitation by bruteforce (i.e. just by increasing the plasma power) a practical limitationis experienced in that high power H₂ noble gas plasmas etch the plasmatube releasing (for the usual case of quartz or pyrex tubes) silicon,oxygen, and a variety of trace contaminants that travel downstream withthe gas flow and are incorporated in the growing diamond film.

A comprehensive solution to the problem involves an important extensionof the concept of remote region of activation, that is, the use ofmultiple remote regions of activation. In this specific case twoseparate plasma tubes are operated. One with a H₂ gas flow and plasmaconditions optimized for H atom production, and one with a noble gasflow (e.g., He) with plasma conditions optimized for metastableproduction. This scheme avoids the tube errosion and contaminationproblems noted in Case 2 above, because mixed H₂, noble gas plasmas arenot formed and H₂ plasma power densities are significantly reduced(reduction factor>10). At the same time the separate H₂ plasma is moreefficient (by orders of magnitude) in the creation of atomic hydrogen ascompared to the metastable activated scheme (Case 1 above). In additionsince the cross section for energy exchange between excited metastablenoble gas species and atomic hydrogen is much less (order of magnitude)then the same cross section for molecules H₂, the multiple remoteregions of activation concept allows both higher deposition rates(rate˜7 Å/sec) and a much broader range of accessible effective H:CH₄ratios than Case 1 above, thus allowing improvements both in filmquality and deposition rate.

It should be noted that this is one specific example of the broadconcept of multiple remote regions of activation, where such regions candiffer in many ways including source material (e.g., gas feed as in theexample above), means of activation (RF, thermal, etc.) spatial filterdesign, and so on. Indeed, the multiple activation region conceptpermits optimization of spatial filter designs which for mixed gassources otherwise would of necessity involve design compromises based onthe differing criteria for exclusion of the unwanted reactive species oftwo different parent gases.

Further commenting on the spatial filtering employed according to thepresent invention, the design of the spatial filter in large partprovides the flexibility in the remote region of activation scheme.First, it is noted that the region of spatial filtration works in bothdirections. First it is optimized to transmit the desired excitedspecies from the region of activation (e.g., R.F. plasma) to the mixingregion while suppressing the transmission of other excited speciescreated in the region of activation. Second and equally important, itsupports the remote aspect of a region of activation by allowing thesuppression of back-diffusion of any species present in the depositionchamber. Thus without a spatial filter the region of activation cannotbe remote.

In order to realize the flexibility of the scheme, it is important torealize that the spatial filter is not simply a time delay where merelythe intrinsic lifetime of the various reactive species is allowed tochange the relative distribution of reactive species at different planes(times) downstream of the region of activation. If this was true, thetechnique would be restricted to enhancing only the intrinsically longerlived species and would be unable to separate species with nearly equalintrinsic lifetimes.

Fortunately the intrinsic lifetime is only one component of the crucialparameter, the effective lifetime. The effective lifetime can becontrolled in many cases. Consider the example presently disclosedherein for epitaxial growth of semiconductors (Si, Ge, C), oxygen andoxygen containing gases (e.g., H₂ O) are common contaminants of feedgases such as He. Oxygen is readily activated to a metastable state inan R.F. plasma. In addition metastable oxygen species have lifetimes farlonger (factor of 100) than noble gas metastables. Thus a small amountof oxygen or oxygen containing contaminants in the feed gas will have adisproportionately large effect on the growing film. However byincorporating a wall element 16₁ providing an aluminum wall surface in aportion of the spatial filter we effectively quench the metastableoxygen while having no appreciable effect on the metastable noble gasflux. Wall interactions are extremely important (dominate at pressure<10Torr) in determining effective lifetimes, thus flow dimensions andmaterials of construction of the spatial filter can be used to engineerthe relative effective lifetimes and the resultant transmissioncharacteristics of the spatial filter. Note that in this case (oxygen innoble gas) the spatial filter is designed to eliminate the normallylonger lived species.

An additional way to design the spatial filtering selectivity is toinsert a baffle plate 16₂ downstream of the plasma region generallymidway in the spatial filtering region to increase the back pressure. Inthis way, those reactive species which have effective lifetimes whichdecrease as a function of pressure will have a greater susceptibility toannihilation. By locating the baffle plate 16₂ generally midway in thespatial filtering region, sufficient forward flow of feed gas reactivespecies is produced to prevent back-diffusion.

Because of the importance of wall interactions, the downstream end 16₃of the spatial filter tubulation is normally extended through thechamber wall. Thus, the wall material of choice for gas streaminteractions is maintained, independent of the considerations governingthe selection of deposition chamber material. In the interior of thedeposition chamber, the large increase in cross section relative to thespatial filter tubulation 16₃ and the flow pattern resulting from thehigh velocity gas stream introduced through the spatial filter exitplane minimize any effect of wall interactions in the depositionchamber. This design feature is referred to as a reentrant sourcetubulation. It is shown in FIG. 2.

One of the foremost problems in low temperature chemical vapordeposition is the removal of hydrogen from the nucleating film. For CH₄the spontaneous desorption of hydrogen occurs around 1000° C. For SiH₄it is around 500° C. For GeH₄ it is around 350° C. Growth of diamond,silicon, or germanium below these respective temperatures then requiressome other process besides thermal desorption to rid the depositedlayers of hydrogen. One approach is to supply the surface which someother source of energy, photons, electrons, ions, etc.

According to the present invention, a flux of metastables is supplied tothe deposition surface. The same metastables which dissociate bonds inthe gas phase can liberate hydrogen bonded on the nucleating solid. Thisis accomplished by keeping the carrier gas concentrations low to preventtotal quenching of the metastable in the reaction zone. However, becausequenching of the metastables is necessary to form precursor specieswhich can deposit at lower temperatures, there is a compromise madebetween the deposition rate determined by how many metastables arequenched and the dehydrogenation rate determined by how many metastablessurvive the reaction zone and are incident on the substrate.

One approach developed according to the present invention uses asequentially pulsed growth technique where one deposits for somedeterminant period of time with the carrier gas flowing, removes thecarrier gas, and dehydrogenates for some determinant period of time. Thegrowth period is sustained long enough to deposit a monolayer ofmaterial. The dehydrogenation period is sustained long enough to rid thedeposited monolayer of hydrogen. Because the carrier gasses are notquenching the metastable flux, the metastable flux the surface would bemaximum and the dehydrogenation time minimized. One might also expectthat the metastable flux to the surface to impart energy to the adsorbedatoms and increase their mobility on the growth surface. In general, thehigher the surface atom mobility is, the better the crystal will grow.The following are key operating parameters of the pulsed growth sequencetechnique of the present invention:

Deposition Sequence

Ar 200 sccm plasma tube

Ar 50 sccm ring feed

SiH₄ 10 sccm ring (2% SiH₄ in He)

Pressure 0.200 Torr

rf power 40 W

substrate temperature 200° C.

deposition time 1 min

Dehydrogenation Sequence

Ar 200 sccm plasma tube

Ar 50 sccm ring feed

SiH₄ O sccm ring

Pressure 0.200 Torr

rf power 40 W

substrate temperature 200° C.

dehydrogenation time 30 sec

The overall sequence includes repeated alternate performances of theabove noted deposition and dehydrogenation sequences.

Remote plasma enhanced chemical vapor deposition, RPECVD, according tothe present invention, avoids the problems associated with conventionalPECVD techniques. There are three primary differences between RPECVD andPECVD. First, the parent gas molecules are not excited in the plasmaregion but instead react with excited, metastable gas species that flowfrom the plasma region. These metastable species have well-definedmetastable energy states that 4-20 eV above their ground state,depending upon which noble gas is used. By selecting the appropriatenoble gas, it is possible to tune the energy increment used in themetastable specie. The coupling of this energy into the parent moleculesduring collisional events determines the gas phase species. The plugvelocity of gas through the plasma tube prohibits back-diffusion ofparent molecules into the plasma region. Because there is a fairlylimited number of collisional by-products, the RPECVD process offersmore control the type of species that is incident on the growth surfaceduring a deposition. Table I shows a list of the collisional by-productsobtained when different noble gas metastables collide with methane (seeBolden et al., J. Phys. B.: 3,71 (1970) and Balamuta et al, J. Chem.Phys. 79,2822 (1983)), as follows:

                  TABLE I    ______________________________________    NOBLE    GAS SPECIES   BY-PRODUCT    ______________________________________    He            CH.sub.4.sup.+, CH.sub.3.sup.+, CH.sub.2.sup.+    Ar            CH.sub.3, CH.sub.2    Xe            CH.sub.3    ______________________________________

A second primary difference between PECVD and RPECVD is that in RPECVD,unlike in conventional PECVD, the substrates are well removed from theplasma region, minimizing the plasma densities near the substrate. Thisshould result in virtually no sheath fields between the substrate andthe plasma in contrast to immersion systems. Ions created by Penningprocesses in the vicinity of the substrate see no large sheath fields toaccelerate them. Furthermore, with typical deposition pressures between100 and 300 mTorr, the ions are thermalized, reducing their incidentenergy on the substrates. Considering the damage and embedment that hasbeen observed in silicon from even moderately low energy ions (<50 eV),reduction of ion flux and energy is certainly an advantage offered byRPECVD. This feature allows extremely low deposition temperatureunconstrained by annealing considerations.

A third difference between the disclosed RPECVD apparatus to growepitaxial layers and typical PECVD apparatus is the ultra-high vacuumcapability of the RPECVD apparatus. As explained below, this ultra-highvacuum capability of the RPECVD, base pressure less than 5×10⁻¹⁰ Torr,is required to obtain epitaxial layers of sufficient quality forelectronic device applications. On the other hand, the PECVD apparatushas a base pressure typically of 1×10⁻⁶ Torr, and never better than1×10⁻⁸ Torr. These base pressures for the PECVD systems, are completelyinadequate to grow epitaxial semiconductor layers.

Important features of the RPECVD epitaxial growth process that clearlydistinguish it from the SiO₂ RPECVD deposition process are:

1. In RPECVD epitaxial growth, no element of the deposited layers passesthrough the plasma region. In the SiO₂ process, O is activated by theplasma and becomes incorporated in the deposited layers.

2. Furthermore, the reaction between the metastable species exiting theplasma tube and the reactant injected by the gas dispersal ring is verydifferent in the two examples. For RPECVD epitaxial growth, themetastable-reactant interaction results in the dissociation of thereactant or parent group. In the case of SiO₂ deposition, thisinteraction results in the formation of chemical groups.

3. The effective lifetimes and, therefore, the pathlengths of themetastable species exiting the plasma tube are quite different in thetwo cases: for metastable oxygen the pathlength is 1-2 meters, whereas,for metastable He*, the pathlength is a few centimeters.

4. The Si epitaxial material is a crystalline material which needslong-range order to obtain good semiconducting properties. Conversely,the dielectric material SiO₂ is an amorphous material, and, therefore,has no long-range or crystalline order. In RPECVD epitaxial growth,metastable He* promotes long-range order through increasing the surfacemobility of the adatoms. For SiO₂ deposition, metastable O* promotes thefurther oxydation of disiloxane adsorbed on the substrate surface, whichreduces the surface mobility of adatoms and enhances the formation ofamorphous material.

While these differences are illustrated for RPECVD of epitaxial Si andamorphous SiO₂, they apply to the general techniques of RPECVD of singlecrystal epitaxial layers of semiconductor materials and RPECVD ofamorphous dielectric materials. Because these differences between thetwo deposition processes are quite fundamental, they therefore requiredifferent reaction chamber designs, especially with respect tocross-section, distance from the plasma tube to the substrate and plugvelocity, and process parameters, including pressure, flow velocity,excitation levels and reactant concentrations.

In any type of epitaxial process the order and cleanliness of thestarting surfaces are of paramount importance. This is especially truein any low temperature epitaxial process where adsorbed atoms may nothave enough mobility unless the energy is provided by some other sourceother than thermal. In the RPECVD apparatus and process according to theinvention, two species from the plasma region to clean substrates ofresidual contaminants have been developed. The first technique involvesdissociation of molecular hydrogen in the plasma region and transport ofatomic hydrogen to the substrate surface. There the hydrogen reacts withresidual carbon and oxygen atoms forming volatile compounds which leavethe surface. Typical operating conditions for this process are 80-100sccm H₂ plasma, 4-5 mTorr, 35 Watts, 300° C. substrate temperature, and20 scc time duration. Because atomic hydrogen may react with the glasswalls of the plasma tube, this process has been refined. Now, metastablespecies of Ar, generated in the plasma region 14, interact with hydrogenflowing from the ring feed 18 to form atomic hydrogen. The plug velocityof the Ar is kept high to prevent hydrogen from back-diffusing into theplasma region. Typical operating conditions for the refined cleaningprocess are 200 sccm Ar plasma, 50 sccm H₂ ring, 100 mTorr, 50 Watts,300° C. substrate temperature, and 30 s time duration. Here as before,the atomic hydrogen reacts with the residual contaminants on thesubstrate to form volatile compounds and leave the surface. Withoutthese effective hydrogen cleaning procedures, none of the epitaxial workwould be possible. As with the epitaxial growth, it is the flux ofparticular selected species from the excitation region to the substratethat is the key to these processes.

Following use of the cleaning procedure and before limitation of theepitaxial growth process it is very important to preventre-contamination of the substrate surface by undesirable gas specieswhich constitute the background or "base" pressure of the growthapparatus. These contaminants can impinge the substrate from a varietyof sources including the several surfaces present in the growthapparatus. Therefore, it is very important to minimize the concentrationof these contaminant species in the apparatus through maintaining a verylow base pressure, less than 5×10⁻¹⁰ Torr. Base pressures larger thanthis value, through contamination and disruption of the crystallineorder of the cleaned substrate surface, will significantly degrade thequality of the epitaxial grown layer For example, at a base pressure of5×10⁻¹⁰ Torr, 10 percent of the substrate surface, will be covered withnew contaminants after only 4 minutes. Yet, a 10 percent surfacecontamination will create an unacceptably large crystalline point defectdensity between 10²⁰ and 10²¹ in the epitaxial layer. Consequently, therequirement of base pressure less than 5×10⁻¹⁰ Torr is a minimumrequirement.

It should further be understood that while the RPECVD technique of theinvention employs a feed gas activation region which certainly isphysically remote from the deposition region, it is also chemicallyremote because back-diffusion into the activation region is preventedand only selected activated gas species such as the metastable noble gasspecies arrive at the substrate surface.

Other considerations important to the present RPECVD apparatus andmethod growth of epitaxial layers of semiconductor materials include:

(1) Design of an ultra high vacuum, pressures less than 5×10⁻¹⁰ Torr,reaction vessel to maintain surface cleanliness after the in situcleaning procedures.

(2) Incorporation of RHEED equipment into the reactor design to allowqualification of the surfaces prior to and after the cleaning proceduresand/or the depositions.

(3) Use of a reentrant plasma tube to eliminate metal exposure to theplasma environment. It is well known that metals in active plasmaenvironment become eroded leading to metal contamination.

(4) Installation of a complete vacuum system bakeout system from thepump-mouths to the gas bottles. This allows system base pressures to beless than 5×10⁻¹⁰ Torr.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A chemical vapor deposition apparatus forgrowing a layer on a substrate, comprising:activation means foractivating a feed gas by increasing the chemical reactivity of said feedgas, including means for introducing said feed gas to an inlet of saidactivation means at a predetermined flow velocity, means for impartingenergy to said feed gas in an activation region thereby producing anactivated feed gas composed of plural reactive species, and an exitplane from which the activated feed gas exits the activation region;spatial filtering means in communication with said exit plane forpassing only selected of said reactive species of activated feed gasbased on the lifetime of said selected of said reactive species; adeposition chamber located downstream of and in communication with saidspatial filtering means, comprising, carrier gas introduction means forintroducing a carrier gas including a constituent element of the layerto be deposited at a predetermined flow velocity into a carrier gasmixing region where said carrier gas is mixed with said selectedreactive species of said activated feed gas to transfer energy from saidselected activated species and partially dissociate and activate saidcarrier gas into plural reactive species of said carrier gas, adeposition region in which said substrate is positioned locateddownstream from said carrier gas mixing region; wherein the flow ratesof said feed and carrier gases are selected so that no back-diffusion ofcarrier gas and reactive species of said feed and carrier gases upstreamof said exit plane occurs; wherein selected reactive species of theactivated feed gas activate the carrier gas and form activated speciesof the carrier gas, the activated carrier gas species being ordered andfurther dissociated through subsequent reactions with said selectedreactive species of the activated feed gas on said substrate.
 2. Anapparatus according to claim 1, comprising:means for maintaining saiddeposition region at a pressure between 1-1000 mTorr; and means formaintaining said substrate at a temperature of 200°-850° C.
 3. Anapparatus according to claim 1, wherein:said feed gas comprise He; andsaid feed gas introducing means and said carrier gas introducing meansintroduce said feed gas and said carrier gas, respectively, at flowvelocities so that the reactive species of activated feed gas passed bysaid spatial filtering means comprises metastable He(2³ S).
 4. Anapparatus according to claim 1, wherein said activation meanscomprises:a reentrant plasma tube coupled to said feed gas introducingmeans via said inlet; wherein said energy imparting means imparts energyto said feed gas in a first portion of said plasma tube.
 5. An apparatusaccording to claim 4, wherein said spatial filtering means comprises asecond portion of said reentrant plasma tube downstream of said firstportion.
 6. An apparatus according to claim 1, wherein said feed gascomprises plural distinct gas constituents, comprising:a plurality ofsaid activation means, each comprising, a respective inlet, feed gasintroducing means for introducing at least a respective one of saiddistinct feed gas constituents to said respective inlet, and energyimparting means for imparting energy to the at least one respective feedgas constituent; and a plurality of said spatial filtering means each incommunication with the exit plane of a respective activation means forpassing only selected of the feed gas reactive species produced in therespective activation means, each of said spatial filtering meanscommunicating with said mixing region of said deposition chamber.
 7. Anapparatus according to claim 6, wherein said feed gas comprises hydrogenand a noble gas, each of which is activated in a respective activationmeans.
 8. An apparatus according to claim 1, wherein said spatialfiltering means comprises a wall element made of a material whichquenches predetermined reactive species having an intrinsic lifetimelonger than that of the selected reactive species that are passedthrough said spatial filtering means.
 9. An apparatus according to claim1, wherein said spatial filtering means comprises:a baffle forincreasing pressure within at least a portion of said spatial filteringmeans, thereby to annihilate feed gas reactive species having effectivelifetimes which decrease as a function of pressure.
 10. An apparatusaccording to claim 8, wherein said spatial filtering means comprises:abaffle for increasing pressure within at least a portion of said spatialfiltering means, thereby to annihilate feed gas reactive species havingeffective lifetimes which decrease as a function of pressure.